I bought my wife a 2007 Ford sport trac about six months ago. It has always made a very peculiar sound and i would like to know if anyone has ever come across the same thing. It only makes this noise when you are driving about 50 mph uphill. The sound is rhythmic and coming from the rear of the truck. I would compare it to driving over sets of rumble strips but quieter. The vehicle is 4x4 and my theory is that it has something to do with this. Please let me know what you think.